The present invention relates generally to a compression control system for an internal combustion engine and control method therefor for controlling charge pressure in an engine combustion chamber. More specifically, the invention relates to a knock-free engine compression control system for avoiding possible engine knocking by controlling the charge pressure in the combustion chambers of a reciprocating internal combustion engine.
One way to obtain better performance in an internal combustion engine and to achieve better fuel economy is to increase the compression of the air/fuel mixture in the combustion engine. Recently, the compression ratio in the engine combustion chambers has tended to be increased for the sake of better engine performance and fuel economy. Such high-compression-type engines experience problematic engine knocking, since increasing the compression in the combustion chambers causes a higher probability of occurrence of engine knocking. In particular, at relatively high engine loads or at substantially high engine temperatures engine knocking at harmful intensity levels may occur. On the other hand, if the engine compression ratio is lowered, the output torque of the engine, and thus the engine output performance, is also decreased. This accordingly degrades fuel economy.
In order to prevent the engine from knocking at harmful intensities at relatively high compression ratios, the spark advance angle can be controlled to retard the spark advance angle in response to the engine knocking. This tends to drive the spark ignition timing in the engine away from the best torque timing, thereby degrading engine output performance. Furthermore, retardation of the spark advance angle may cause an increase in the exhaust gas temperature so that a catalytic converter in the exhaust system may be damaged by the heat of the exhaust gas.
In another approach, the compression ratio in the combustion chamber can be controlled to eliminate engine knocking. This has been attempted by controlling the operational timing of an intake valve, which control has been disclosed, for example, in Japanese Utility Model Publication (Jikko) Sho 52-1953. The compression ratio control has also been attempted by means of a dual compression piston as set forth in Japanese Patent First Publication (Tokkai) Sho 56-46163.
Although various attempts have been made for knock-free compression control in the internal combustion engine, none of the conventional art can successfully and satisfactorily control the compression ratio in accordance with the engine operating conditions.
The present invention is, therefore, intended to provide a engine compression control system and method which satisfactorily and successfully prevent the engine from knocking harmfully without causing a decrease in the engine output performance, therey maintaining fuel economy at a reasonable level. According to the present invention, compression control can be performed by feeding back the introduced air/fuel mixture to an induction system downstream of a throttle valve at a controlled rate which is determined in accordance with the engine operating conditions.